The present invention relates generally to a semiconductor device, and more particularly to a semiconductor device having a zener diode.
In a semiconductor device, such as for example, a semiconductor integrated circuit, a zener zap method is employed, for example, as a method of trimming a circuit constant. The zener zap method trims a circuit constant in the following manner. First, a zener diode is inserted into an integrated circuit such that a reverse bias can be applied the zener diode, and an open circuit portion is constructed in which a leak current flows only through the zener diode. The zener diode is reverse biased, i.e., applied with a high current pulse of several tens of microamperes (mA) as a reverse overcurrent, as required, to short or destroy the zener diode between the anode and the cathode thereof to bring the open circuit portion into a conductive state, thereby trimming the circuit constant.
The zener diode may be formed, for example, from an npn transistor, the emitter region of which serves as a cathode region, and the base region of which serves as an anode region. A metal layer of Al or Al alloy containing approximately 1% of Si, Cu or the like is ohmic contacted on each of these regions to form a cathode electrode and an anode electrode for the zener diode.
The destructively shorted zener diode causes the above-mentioned reverse overcurrent to conduct through a p-n junction between the anode and the cathode thereof, resulting in an increased temperature. Further, a local temperature rise, i.e., the occurrence of a hot spot due to non-uniform distribution of impurities, crystal defects, non-uniform heat dissipation, and so on causes current concentration which gives rise to an instantaneous and local temperature rise as well as the occurrence of a low resistance region and a destroyed junction associated therewith. The migration of Al atoms from the electrodes caused by a subsequent current due to the destroyed junction forms a filament between the anode and the cathode on the surface of the semiconductor, creating short-circuit between the anode and the cathode.
In recent years, by the way, electrodes and wires have been increasingly thinner in response to demands for higher density and further reduction in size of a semiconductor integrated circuit, and correspondingly, each semiconductor region of an electrode in a circuit element thereof, for example, a transistor, is required to have a lower contact resistance. With the required reduction in contact resistance, when an electrode metal layer made of Al or an alloy including Al is used as an electrode, it is necessary to avoid an increased resistance due to an reaction of Al with Si contained in a semiconductor during a heat treatment or the like in a semiconductor manufacturing process. For this purpose, a so-called barrier metal layer made of a refractory or high melting point metal such as, for example, Ti, TiON or the like acts as an underlying layer for the electrode to eliminate the reaction.
However, if the electrode having the arrangement mentioned above is applied to a zener diode, the barrier metal tends to prevent Al from migrating into a semiconductor in destructive short and adversely affects the formation of a filament between the cathode and the anode. Therefore, an extremely large pulse current is required for the destructive short. In addition, a filament thus formed tends not to have a sufficiently reduced resistance.
To eliminate the inconvenience mentioned above, a zener diode has been proposed in which the formation of an underlying metal layer with a barrier metal layer is avoided, for example, only in an anode electrode of the zener diode.
However, when part of the electrodes is formed without the barrier metal structure while the remaining electrodes are formed with the barrier metal structure, the number of manufacturing steps thereof is increased so that the mass productivity is degraded.